The present invention relates generally to cytokine receptors and more specifically to tumor necrosis factor receptors.
Tumor necrosis factor-α (TNFα, also known as cachectin) and tumor necrosis factor-β (TNFβ, also known as lymphotoxin) are homologous mammalian endogenous secretory proteins capable of inducing a wide variety of effects on a large number of cell types. The great similarities in the structural and functional characteristics of these two cytokines have resulted in their collective description as “TNF.” Complementary cDNA clones encoding TNFα (Pennica et al., Nature 312:724, 1984) and TNFβ (Gray et al., Nature 312:721, 1984) have been isolated, permitting further structural and biological characterization of TNF.
TNF proteins initiate their biological effect on cells by binding to specific TNF receptor (TNF-R) proteins expressed on the plasma membrane of a TNF-responsive cell. TNFα and TNFβ were first shown to bind to a common receptor on the human cervical carcinoma cell line ME-180 (Aggarwal et al., Nature 318:665, 1985). Estimates of the size of the TNF-R determined by affinity labeling studies ranged from 54 to 175 kDa (Creasey et al, Proc. Natl. Acad. Sci. USA 84:3293, 1987; Stauber et al., J. Biol. Chem. 263:19098, 1988; Hohmann et al., J. Biol. Chem. 264:14927, 1989). Although the relationship between these TNF-Rs of different molecular mass is unclear, Hohmann et al. (J. Biol. Chem. 264:14927, 1989) reported that at least two different cell surface receptors for TNF exist on different cell types. These receptors have an apparent molecular mass of about 80 kDa and about 55–60 kDa, respectively. None of the above publications, however, reported the purification to homogeneity of cell surface TNF receptors.
In addition to cell surface receptors for TNF, soluble proteins from human urine capable of binding TNF have also been identified (Peetre et al., Eur. J. Haematol. 41:414, 1988; Seckinger et al., J. Exp. Med. 167:1511, 1988; Seckinger et al., J. Biol. Chem. 264:11966, 1989; UK Patent Application, Publ. No. 2 218 101 A to Seckinger et al.; Engelmann et al., J. Biol. Chem. 264:11974, 1989). The soluble urinary TNF binding protein disclosed by UK 2 218 101 A has a partial N-terminal amino acid sequence of Asp-Ser-Val-Cys-Pro-, which corresponds to the partial sequence disclosed later by Engelmann et al. (1989). The relationship of the above soluble urinary binding proteins was further elucidated after original parent application (U.S. Ser. No. 403,241) of the present application was filed, when Engelmann et al. reported the identification and purification of a second distinct soluble urinary TNF binding protein having an N-terminal amino acid sequence of Val-Ala-Phe-Thr-Pro- (J. Biol. Chem. 265:1531, 1990). The two urinary proteins disclosed by the UK 2 218 101 A and the Engelmann et al. publications were shown to be immunochemically related to two apparently distinct cell surface proteins by the ability of antiserum against the binding proteins to inhibit TNF binding to certain cells.
More recently, two separate groups reported the molecular cloning and expression of a human 55 kDa TNF-R (Loetscher et al., Cell 61:351, 1990; Schall et al., Cell 61:361, 1990). The TNF-R of both groups has an N-terminal amino acid sequence which corresponds to the partial amino acid sequence of the urinary binding protein disclosed by UK 2 218 101 A, Engelmann et al. (1989) and Englelmann et al. (1990).
In order to elucidate the relationship of the multiple forms of TNF-R and soluble urinary TNF binding proteins, or to study the structural and biological characteristics of TNF-Rs and the role played by TNF-Rs in the responses of various cell populations to TNF or other cytokine stimulation, or to use TNF-Rs effectively in therapy, diagnosis, or assay, purified compositions of TNF-R are needed. Such compositions, however, are obtainable in practical yields only by cloning and expressing genes encoding the receptors using recombinant DNA technology. Efforts to purify the TNF-R molecule for use in biochemical analysis or to clone and express mammalian genes encoding TNF-R, however, have been impeded by lack of a suitable source of receptor protein or mRNA. Prior to the present invention, no cell lines were known to express high levels of TNF-R constitutively and continuously, which precluded purification of receptor for sequencing or construction of genetic libraries for cDNA cloning.